In the paper ‘A deadly cocktail’ we describe the results of a multi-proxy high-resolution analysis of a sediment core from the Mare aux Songes (MAS), a coastal wetland on the Indian Ocean island of Mauritius. Here, a rich multi-taxic fossil site was discovered in 2005. The 4200-yr old sediment deposits represent a Lagerstätte where 100,000s of vertebrate individuals, including the extinct Dodo (Raphus cucullatus) and extinct Giant Tortoises (Cylindraspis spp.), died within a time frame of less than 150 years; long before the Dutch first colonized the island in AD 1638.

An interdisciplinary, international team of researchers collaborated under the banner of the DodoAlive Foundation to examine in fine detail the impact of this natural catastrophe on an island ecosystem. Their aim was to answer the following questions: How was this fossil site formed?, under which conditions did so many vertebrates die, and what was the ultimate cause of the mass mortality events at MAS?

Analyses of pollen, diatoms, XRF geochemistry and pigments provide a unique window into the MAS ecosystem before and during the mass mortality events. Our data show how a major drought 4200 years ago turned the freshwater at MAS into a deadly cocktail. This prolonged dry period was triggered by the onset of the El Nino-Southern Oscillation climate regime (De Boer et al., 2014). The drought regionally induced fires on Mauritius and limited freshwater sources like MAS by decreasing local water-levels increasing salt levels. The excrements of the vertebrates produced hypertrophic conditions that, combined with salinization and high temperatures, created a suitable environment for toxic cyanobacteria, of which the pigments were found. These factors led to a deadly cocktail, resulting in the death of 100,000s of vertebrates by intoxication, dehydration, trampling and miring.

The fossil site provides a unique window into the response of an insular ecosystem to an extreme climatic event. Life on islands was not always a paradise for those species that relied on a stable source of freshwater. During times of climatic instability and drought, non-territorial species, such as flamingos, ducks, and pigeons, could move to other parts of the island with a different micro-habitat. However, flightless and territorial freshwater-dependent species such as the Dodo and Giant Tortoise were tied to the local water holes and may have experienced population collapse from climatic bottlenecks. The persistence of these species on the island throughout the Quaternary demonstrates the resilience of these insular biota. The Dodo and Giant Tortoises, however, went extinct amongst dozens of other endemic vertebrates shortly after human colonization. The introduction of rats, pigs, hunting, and large-scale destruction of forest provided tough challenges that almost all of the native fauna were not prepared for.